In recent years, there have been proposed and developed various toroidal continuously variable transmissions, often abbreviated to “TCVTs”, employing two different ratio control hydraulic systems, namely forward and reverse ratio control hydraulic systems. Japanese Patent Provisional Publication No. 8-93873 (hereinafter is referred to as JP8-93873) teaches technologies for controlling a TCVT employing both forward and reverse ratio control hydraulic systems, in presence of a reverse ratio control hydraulic system failure during a reverse operating mode, or when the forward ratio control hydraulic system is erroneously used instead of the reverse ratio control hydraulic system during the reverse operating mode. For instance, in presence of the reverse ratio control hydraulic system failure during the reverse operating mode, the control system disclosed in JP8-93873 outputs a command signal to a ratio control actuator associated with a forward ratio control valve so that the transmission ratio shifts toward a high gear ratio (a speed-increase side) while using the forward ratio control hydraulic system. The TCVT has a trunnion serving as a power roller support that rotatably supports a power roller, which is interposed between input and output disks and is in contact with a torus surface of each of the input and output disks under preload. As is generally known, in the TCVT, the trunnion is vertically offset from the common rotation center of input and output disks in one axial direction of its trunnion axis during a forward operating mode. Conversely during a reverse operating mode, the trunnion normally shifts from the common rotation center in the opposite direction of its trunnion axis. That is, shifting the transmission ratio toward the high gear ratio while using the forward ratio control hydraulic system during the reverse operating mode, practically means shifting the transmission ratio toward a low gear ratio. As a result, it is possible to start the vehicle at a low gear ratio even in the presence of the reverse ratio control hydraulic system failure during the reverse operating mode. For the reasons discussed below, the TCVT commonly uses forward and reverse ratio control hydraulic systems. Generally, a trunnion vertical offset versus power-roller tilt angle (correlated to a transmission ratio) characteristic is somewhat unstable. To enhance this characteristic, a so-called precision cam is often provided to mechanically transmit both the tilting motion (a gyration angle or tilt angle of the power roller) of the trunnion and the vertical offset (vertical displacement) of the trunnion via the precision cam to a ratio control valve spool of a ratio control hydraulic system, which serves to feed hydraulic oil to a servo piston depending on a displacement of the ratio control actuator. By way of feedback action of the precision cam, the tilt angle of the power roller and the vertical offset of the trunnion are both fed back to the ratio control valve spool, thereby mechanically stabilizing the trunnion vertical offset versus power-roller tilt angle characteristic. Rotation directions of input and output disks during the forward operating mode are opposite to rotation directions of the input and output disks during the reverse operating mode. As a matter of course, the direction of trunnion tilting motion with respect to the trunnion vertical offset during the forward operating mode is opposite to that created during the reverse operating mode. Thus, one of two different ratio control hydraulic systems, each having a different polarity, is used as a forward ratio control hydraulic system, whereas the other is used as a reverse ratio control hydraulic system. Switching between the forward and reverse ratio control hydraulic systems is achieved by means of a forward/reverse changeover valve.
On the other hand, Japanese Patent Provisional Publication No. 8-178042 (hereinafter is referred to as JP8-178042) teaches technologies for stably driving a ratio control actuator of a continuously variable transmission (CVT) while preventing the ratio control actuator from being large-sized. The CVT control system disclosed in JP8-178042 calculates a desired transmission ratio based on engine/vehicle operating conditions, and brings a ratio control valve spool closer to a stroke position corresponding to the desired transmission ratio by operating the ratio control actuator such as a step motor. Bringing the ratio control valve spool closer to the desired stroke position produces a speed-change control pressure corresponding to the desired transmission ratio. In this manner, the transmission ratio of the CVT can be steplessly varied toward the desired ratio responsively to the speed-change control pressure. However, a viscosity of working fluid used within the CVT varies depending on a working-fluid temperature, such that the viscosity increases as the working-fluid temperature falls. Usually, a valve body of the ratio control valve is made of aluminum alloy, whereas a spool of the ratio control valve is made of iron. The clearance between the valve body and the valve spool tends to reduce, as the working-fluid temperature falls. For the reasons set out above, at low working-fluid temperatures, the viscous resistance against the stroke of the valve spool tends to increase. Therefore, a greater driving force of the ratio control actuator (e.g., a step motor) has to be generated when the CVT is still cold and thus the working-fluid temperature is low. From the fact that it is possible to reduce the required driving force of the ratio control actuator by reducing or decreasing a ratio-control-actuator driving speed for the same working-fluid temperature, in other words, for the same viscous resistance against the stroke of the valve spool, JP8-178042 variably controls the ratio-control-actuator driving speed depending on the working-fluid temperature. This effectively reduces the required driving force of the ratio control actuator even when the CVT is cold, thus preventing the ratio control actuator from being large-sized undesirably.